There are over 100 nuclear power plants in operation in the United States, most of which are Pressurized Water Reactor (PWR) designs. These plants generate approximately 20% of the electricity used in the United States. The reactors operate using fuel made from uranium (U) enriched in the fissile U-235 isotope. The maximum initial U-235 enrichment in new fuel is generally slightly less than 5% (the amount of U-235 in naturally occurring uranium is 0.071%). During its lifetime, the uranium is typically burned to a final enrichment of slightly more than 1.0%. At the end of its useful lifetime in a reactor, the spent nuclear fuel (SNF) is stored temporarily in water pools or in dry canisters and casks. It is possible that some SNF can contain sufficient unburned fissile material that, in the presence of water, an array of SNF assemblies could theoretically experience criticality (i.e., a sustained nuclear chain reaction). Although it is extremely unlikely that a chain reaction would occur, to insure that criticality does not occur, the structures that hold the SNF in spent fuel pools and dry storage canisters and casks are designed and built with criticality control features, including SNF positional or geometry control and neutron absorbing materials, that prevent spontaneous criticality from occurring.
There are a number of spent fuel containment technologies approved by the U.S. Nuclear Regulatory Commission (USNRC) and there are many SNF storage systems in safe operation. Most of the dry storage canisters and casks are designed and licensed as dual-purpose systems (i.e., for storage and transportation). The U.S. Department of Energy (DOE) has issued a specification for a type of canister that can be used not only for storage and transportation, but also for disposal in a geologic repository. The new canister is called the Transportation, Aging, and Disposal (TAD) package. The conceptual design of the TAD package is based on installing a canister inside different overpacks for the transportation, storage, and disposal activities. The TAD canister and internal components remain the same in all of these activities. The specification calls out borated stainless steel plates as the neutron absorber material to be used inside the canister. The specification also defines a 10,000-year design lifetime.
There will be at least three barriers to protect the spent fuel in the repository—the canister overpack, the TAD canister shell, and the fuel cladding (typically Zircaloy). In the DOE TAD design, each fuel assembly is surrounded on all four sides by a borated stainless steel plate. A basket structure is designed to hold the borated stainless steel plates in place and create a neutron flux trap between the absorber plates to increase the effectiveness of the neutron absorber. A design requirement of the TAD specification for the neutron absorber material is that the material have sufficient corrosion resistance that the absorber effectiveness is maintained after exposure to groundwater for a period of 10,000 years. The borated stainless steel plates meet this requirement by having a thickness that includes a corrosion allowance. It is postulated that over thousands or tens of thousands of years, these barriers will be breached by corrosion from groundwater, and the groundwater will penetrate the TAD canister. The presence of groundwater in the TAD canister provides moderation of the neutrons in the system and could lead to criticality, if there is insufficient criticality control in the form of neutron absorbing material.
Accordingly, there is a need for a neutron absorbing material that will assure that the SNF maintains subcriticality after the degradation of the multiple barriers and the ingress of groundwater into the TAD canister.